The present invention relates to a disc defect inspection apparatus for use in inspecting a defect of a disc, such as an optical disc or a magneto-optical disc in and from which data is recorded and reproduced optically.
When optical discs in and from which data is optically recorded and reproduced by laser light are manufactured, the manufactured optical discs are inspected in order to see whether a signal recording surface or a disc surface is formed in the predetermined state (i.e., the signal recording surface or the disc surface is smooth). When the optical disc is inspected, laser light is radiated on the signal recording surface of the optical disc, for example, and an intensity of reflected light of the laser light is detected. If the intensity of reflected light exceeds a predetermined amount, then it is determined that the signal recording surface has a certain defect.
According to the above-mentioned method of inspecting a defect of an optical disc by detecting an intensity of reflected light, a defect of the optical disc can be detected, however, the state in which a defect has occurred in the optical disc cannot be specified closely. Specifically, even though some defects, such as irregularities formed on the signal recording surface of the optical disc can be judged according to the above-mentioned method, types of irregularities, i.e., concavities or convexities cannot be specified. Moreover, heights of convexities or depths of concavities cannot be detected.
On the other hand, there are known some different types of defects (unevenness on the disc surface) wherein a shape of a beam spot is distorted even though an intensity of reflected light is not changed substantially. These types of defect cannot be detected.
Concavities and convexities formed on the disc surface can be detected by increasing a sensitivity at which an intensity of reflected light is detected (i.e., defect can be judged on the basis of a very small change of an intensity of reflected light). However, if the detection sensitivity increases as described above, then a smudged disc surface may be determined as a defect of the disc. There is then the risk that a good disc will be erroneously determined as a defective disc.
To solve the aforesaid problem, the following method is proposed. All data that are recorded on one disc, for example, are previously stored in a memory. All data read out from an inspected disc are compared with the data stored in the memory one after another, thereby making it possible to completely inspect each inspected disc. However, when the above-mentioned inspection is carried out, an inspection apparatus needs a memory having a large storage capacity. Furthermore, the inspection requires a lot of time and is not suitable in actual practice.